Locomotor systems of the frog undergo dramatic morphological and functional change during metamorphosis. The research will investigate the contribution of changing sensory inputs, neuropharmacological change, and hormonal events on development of locomotor behavior and its underlying neural circuitry. The basic hypothesis is that the interneuronal network producing rhythmic undulations of the tadpole is conserved, at least in part, to coordinate the hindlimbs of the postmetamorphic frog. Mechanisms underlying pedal locomotion and undulatory swimming coexist throughout much of the larval period, even though hindlimb locomotor behavior is not normally observed until metamorphic climax. Preliminary results suggest that hindlimb locomotor behavior is suppressed by descending inputs prior to metamorphosis. Therefore, research will focus on factors that experimentally facilitate activity of the hindlimbs in tadpoles and could normally "turn on" pedal locomotion at metamorphosis. This activation of hindlimb motoneurons could be accomplished by specifically gating the of output of locomotor pattern generators to hindlimb motoneurons or by non-specifically increasing the excitability of hindlimb motoneurons. The problem will be attacked behaviorally by using single-frame analyses of locomotor behavior. It will be attacked electrophysiologically by recording, in vitro, the output of motoneurons innervating larval axial muscles and motoneurons innervating the hindlimbs, using an isolated central nervous system preparation. Answers to these questions may lead to a better understanding of mechanisms of locomotor development in other limbed vertebrates, including mammals. This research may also yield insight into general mechanisms of motor development. Are the same circuits used to produce different, functionally-related behaviors at different times during development? For example, are circuits that underlie suckling behavior of the infant later used to produce mastication? If so, what are the factors that determine whether those circuits should produce sucking or mastication? Why does brain damage in adulthood sometimes result in the return of the rooting reflex and sucking responses?